Functionalized azobenzenes for micellar solar thermal energy storage as a next-generation MOST system – Nature

Executive Summary

This report details the development of a Micellar Solar Thermal Energy Storage (MIST) system, an innovative approach designed to address critical challenges in solar energy capture and storage, directly contributing to the United Nations’ Sustainable Development Goals (SDGs), particularly SDG 7 (Affordable and Clean Energy). While solar energy is the most abundant sustainable resource, its intermittent nature necessitates efficient long-term storage solutions. Conventional Molecular Solar Thermal Energy Storage (MOST) systems, often based on azobenzene molecules, are limited by low energy density, short storage durations, and a reliance on organic solvents, which conflicts with SDG 12 (Responsible Consumption and Production). The MIST system overcomes these limitations by utilizing self-assembling azobenzene-functionalized dipeptides in aqueous and gel states. This approach significantly enhances energy storage stability, extending the calculated thermal half-life of the stored energy from 148 days in an organic solvent to an unprecedented 12.8 years in a gel state. The system demonstrates competitive energy densities, superior material processability in water, and macroscopic heat release, marking a significant advancement towards scalable, sustainable, and deployable solar energy technologies.

Introduction: Aligning Energy Storage with Sustainable Development Goals

The global transition towards renewable energy sources is fundamental to achieving SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action). Solar energy represents a cornerstone of this transition, but its effective utilization is hampered by the challenge of storing energy for on-demand use. Molecular Solar Thermal Energy Storage (MOST) offers a promising pathway by capturing solar energy in the chemical bonds of photoswitchable molecules. However, the widespread adoption of this technology has been constrained by several key challenges:

• Limited Storage Duration: Many systems suffer from rapid thermal relaxation, leading to energy loss over short periods.
• Low Energy Density: The amount of energy stored per unit mass is often insufficient for practical applications.
• Environmental Concerns: A significant reliance on organic solvents raises issues of toxicity and sustainability, undermining the principles of SDG 12 (Responsible Consumption and Production).

This report introduces a novel MIST system that directly addresses these limitations by leveraging self-assembly in aqueous environments, thereby creating a more sustainable and effective platform for solar energy storage.

The MIST System: An Innovative Approach to Sustainable Energy Storage

System Design and Self-Assembly

The MIST system is based on a series of azobenzene-functionalized dipeptide amphiphiles designed to self-assemble into micellar structures in aqueous solutions. By modifying the dipeptide sequence, the morphology of the self-assembled aggregates can be precisely controlled, forming distinct structures as confirmed by small-angle X-ray scattering (SAXS) analysis:

1. Spherical Micelles: Formed by Azo-FI and Azo-FV compounds.
2. Cylindrical Micelles: Formed by the Azo-FL compound.
3. Worm-like Elliptical Cylinders: Formed by the Azo-FF compound.

This controlled self-assembly is the foundational principle that enhances the system’s energy storage capabilities.

Contribution to SDG 12 (Responsible Production)

A primary innovation of the MIST system is its compatibility with water. By operating effectively in aqueous dispersions and gels, this approach eliminates the need for hazardous organic solvents. This shift is a critical step towards developing environmentally benign energy technologies and aligns directly with the goals of sustainable industrial processes and responsible chemical management as outlined in SDG 12. This water-processability also enhances safety and scalability for future industrial applications, contributing to SDG 9 (Industry, Innovation, and Infrastructure).

Performance Analysis and Contribution to SDG 7 (Affordable and Clean Energy)

Enhanced Energy Storage Longevity

The core achievement of the MIST system is the dramatic extension of the energy storage lifetime. The self-assembled structures restrict molecular mobility, which significantly increases the activation barrier for the thermal back-conversion of the energy-storing cis-isomer. This stabilization is progressively enhanced as the system transitions from a solution to a gel state.

• Organic Solvent (DMSO): The calculated thermal half-life is 148 days.
• Aqueous Micellar Dispersion: The half-life increases to 233 days, demonstrating the stabilizing effect of self-assembly in water.
• Aqueous Gel State: The half-life is extended to an extrapolated 4674 days (12.8 years), representing a monumental improvement over existing systems.

This exceptional stability makes the stored solar energy available for long-term, on-demand use, a critical requirement for making solar power a reliable and affordable primary energy source.

Energy Density and Reusability

The system demonstrates practical energy storage capacities, with the Azo-FV compound exhibiting the highest energy release of 151 J g⁻¹, a value competitive with other azobenzene-based systems. Furthermore, the system’s robustness was confirmed through multiple photoswitching cycles. The MIST gel system at a high concentration (200 mg mL⁻¹) was cycled five times with no loss in isomerization efficiency or significant degradation in heat release. This reusability is essential for the economic viability and resource efficiency of any practical energy storage technology, reinforcing its alignment with SDG 7 and SDG 12.

Macroscopic Heat Release and Practical Application

A successful demonstration of on-demand energy release was conducted using a highly concentrated Azo-FV gel. Upon irradiation with light, the stored energy was released as heat, causing a macroscopic temperature increase of up to 5.7 °C. This experiment validates the MIST system’s potential for real-world applications, such as providing clean heat for residential or industrial processes, contributing to the development of sustainable cities and communities (SDG 11).

Conclusion: Future Directions for Next-Generation Sustainable Energy Technologies

Advancements and Scalability

The MIST design strategy represents a significant advancement in the field of molecular solar thermal energy storage. By integrating photoresponsive functionality with dipeptide-driven self-assembly, this work establishes a versatile platform for creating highly stable, water-processable, and effective energy storage materials. The ability to achieve exceptionally long storage durations and demonstrate macroscopic, reversible heat release in an aqueous gel marks a critical step toward developing scalable and deployable technologies that can meaningfully contribute to a global sustainable energy infrastructure.

Future Research and Optimization

While the current system demonstrates remarkable performance, future work will focus on further optimization to enhance its practical utility and alignment with global sustainability targets. Key areas for future research include:

1. Improving Solubility: Modifying the molecular structure to improve solubility at high concentrations will enhance light penetration and overall photoconversion efficiency.
2. Enhancing Thermal Stability: Incorporating steric hindrance into the molecular design could further stabilize the energy-storing isomer.
3. Exploring Alternative Solvents: Investigating environmentally friendly solvents with lower heat capacity, such as ethylene glycol, may improve system performance.

Through these optimizations, the MIST platform has the potential to become a cornerstone of next-generation energy systems that are responsive, durable, and fundamentally sustainable.

1. Which SDGs are addressed or connected to the issues highlighted in the article?

SDG 7: Affordable and Clean Energy

The article’s primary focus is on developing a more efficient and sustainable method for capturing and storing solar energy. This directly aligns with SDG 7, which aims to ensure access to affordable, reliable, sustainable, and modern energy for all. The research on Molecular Solar Thermal Energy Storage (MOST) systems seeks to overcome the challenges of harnessing solar power, which the article describes as “the most abundant sustainable energy resource.”

SDG 9: Industry, Innovation, and Infrastructure

The development of the Micellar Solar Thermal Energy Storage (MIST) system represents a significant scientific and technological innovation. The article details a novel approach using “micellar aggregates” and “self-assembled gels” to improve energy storage. This contributes to SDG 9’s goal of building resilient infrastructure, promoting inclusive and sustainable industrialization, and fostering innovation.

SDG 12: Responsible Consumption and Production

The article addresses the environmental drawbacks of conventional MOST systems, specifically their “reliance on organic solvents.” The proposed MIST system offers a more sustainable alternative with its “water-compatible formulations.” This shift towards environmentally sounder chemical processes supports SDG 12, which promotes sustainable consumption and production patterns, including the environmentally sound management of chemicals.

SDG 13: Climate Action

By advancing technology for renewable energy storage, the research implicitly supports SDG 13. Efficient solar energy storage is crucial for reducing dependence on fossil fuels, which is a key strategy for mitigating climate change. The article’s goal of creating “scalable, deployable energy storage technologies” contributes to the broader effort of taking urgent action to combat climate change and its impacts.

2. What specific targets under those SDGs can be identified based on the article’s content?

SDG 7: Affordable and Clean Energy

• Target 7.2: By 2030, increase substantially the share of renewable energy in the global energy mix. The research directly supports this target by developing a more viable and efficient technology for solar energy, which is a key renewable source. The article aims to create “next-generation energy storage systems” that can effectively harness solar power.
• Target 7.a: By 2030, enhance international cooperation to facilitate access to clean energy research and technology… and promote investment in energy infrastructure and clean energy technology. This scientific publication itself is a form of knowledge sharing that facilitates access to clean energy research. The development of a “highly stable, and suitable for scalable solar thermal energy conversion” system is designed to attract further research and investment.

SDG 9: Industry, Innovation, and Infrastructure

• Target 9.4: By 2030, upgrade infrastructure and retrofit industries to make them sustainable, with increased resource-use efficiency and greater adoption of clean and environmentally sound technologies. The MIST system is presented as a cleaner technology that avoids the “reliance on organic solvents” of previous systems, making it an environmentally sounder process.
• Target 9.5: Enhance scientific research, upgrade the technological capabilities of industrial sectors… encouraging innovation. The entire article is a demonstration of this target in action, presenting a novel “MIST molecular design strategy” that enhances scientific understanding and upgrades technological capabilities in the energy storage sector.

SDG 12: Responsible Consumption and Production

• Target 12.4: By 2020, achieve the environmentally sound management of chemicals… and significantly reduce their release to air, water and soil. The article’s emphasis on creating “water-compatible formulations” and moving away from organic solvents directly addresses this target by proposing a system with a reduced environmental footprint.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

Yes, the article mentions several specific, quantifiable indicators that can be used to measure the performance and progress of the developed technology.

Indicators for SDG 7 and SDG 9 (Clean Energy and Innovation)

• Energy Storage Duration (Thermal Half-life): This is a key performance metric. The article quantifies the improvement, stating the half-life extends “from 148 days in dimethyl sulfoxide (DMSO), to 233 days in water, and to 12.8 years in the gel state.” This directly measures the technology’s long-term storage capability.
• Energy Density: The capacity for energy storage is measured. The article notes that “Azo-FV exhibited the highest energy release (151 J g⁻¹),” providing a clear indicator of storage efficiency.
• Macroscopic Heat Release: The practical energy output is measured as a temperature change. The system demonstrated a “macroscopic heat release in the gel state (up to 5.7 °C),” which serves as an indicator of its real-world applicability.
• Cyclability and Reversibility: The durability of the system is measured by its ability to be reused. The article states it “can be cycled multiple times with no loss of efficiency,” with data showing the “degree of isomerization is essentially unchanged over 5 cycles.”

Indicators for SDG 12 (Responsible Production)

• Solvent System: The shift from organic to aqueous systems is a key indicator of environmental soundness. The article highlights the development of “water-compatible formulations” and “water-processable” materials as a major advantage over conventional systems that have a “reliance on organic solvents.”

4. Table of SDGs, Targets, and Indicators

Source: nature.com